Reflection densitometer

ABSTRACT

A reflection densitometer comprising a light source for directing a beam of light onto a surface and a light transmission conduit for receiving and transmitting the light reflected from the surface along a predetermined path. The beam of light and the path lie substantially in the same plane. The beam forms a light angle with a reference line and the path forms a reading angle with the reference line. The light angle and the reading angle are maintained within predetermined ranges.

United States Patent 11 1 Hyman et al.

[ 1 Nov. 5, 1974 l REFLECTION DENSITOMETER [751 .lnventors: Maxwell Hyman, El Toro; Donald C.

De Puy, Orange, both of Calif.

[73], Assigncc: Align-O-Tron Corporation, Orange,

Calif.

1221 Filed: Aug. 3, 1972 211 Appl. No.: 277,491

[52] US. Cl. 356/188, 356/210 {51] Int. Cl GOlj 3/48, GOln 21/48 [58] Field Of Search 356/201, 203, 210, 212,

[561 References Cited UNITED STATES PATENTS 2,807.187 9/1957 Petry ,1 356/203 2837965 6/1958 Goldsmith 353/55 2,842.025 7/1958 Craig 356/203 3,473,878 10/1969 Schweitzer 356/210 3,708,233 1/1973 Van Dyk et al. 1 356/244 3,712,745 1/1973 Armstrong et a1 356/244 FOREIGN PATENTS OR APPLICATIONS 684,508. 12/1952 Great Britain 356/186 OTHER PUBLICATIONS A New Reflection Densitometer, H. Nitka & S. W. Stammers, Royal Photographic Society (London), TRI p. 63.

Primary Examiner-Vincent P. McGraw Attorney, Agent, or Firm-Smyth, Roston & Pavitt [57] ABSTRACT A reflection densitometer comprising a light source for directing a beam of light onto a surface and a light transmission conduit for receiving and transmitting the light reflected from the surface along a predetermined path. The beam of light and the path lie substantially in the same plane. The beam forms a light angle with a reference line and the path forms a reading angle with the reference line. The light angle and the reading angle are maintained within predetermined ranges.

17 Claims, 15 Drawing Figures REFLECTION DENSITOMETER BACKGROUND OF THE INVENTION A reflection densitometer is an instrument which is used to measure the optical density of a surface. Reflection densitometers have application, for example, in the printing industry where it is necessary to measure the density of black and colored inks applied to surfaces.

More particularly, a reflection densitometer is sensitive or responsive to light reflected from a surface. The intensity of the reflectedlight can be used to measure the density. Density increases as a surface is made darker. Thus, a black surface is more dense than a gray surface, and a dark yellow surface is more dense than a light yellow surface.

Prior art reflection densitometers are generally not as accurate as desired particularly to relatively high densities. Color densitometers must be capable of detecting very slight color variations, and prior art color densitometers are generally defective in this regard. In addition, reflection densitometers used heretofore are relatively large, expensive, complex, and employ relatively involved optical systems.

SUMMARY OF THE INVENTION The concepts and features of the present invention are applicableto an instrument such as a reflection densitometer which includes means for directing a beam of light onto a surface and means responsive to the intensity of the light reflected along a predetermined path for providing an indication which is a function of the intensity of the reflected light. This invention is based, in part, upon the discovery that performance and accuracy of such an instrument can be materially increased by maintaining the proper orientation of the beam of light, the predetermined path, and the surface. As applied to a reflection densitometer, a quantity known as differential density is increased by following the teachings of this invention. Generally, the higher the differential density, the greater the ability to differentiate between different densities. Thus, the preferred and optimum orientations can be determined by considering differential density.

Another feature of the invention is the ease with which any desired test region can be located and the speed with which the density of such region can be determined. To accomplish this, the reflection densitometer includes a supporting structure and a target carried by the supporting structure. The target has a preselected location which is capable of passing light and which is adapted to be placed over the test region. The reflection densitometer can be moved over the surface until the test region is directly beneath the preselected location of the target. This accurately and rapidly locates the test region.

To determine the density of the test region, a head is moved from a retracted position in which the head leaves the preselected location uncovered to an operative position in which the head overlies the preselected location so that it can direct a beam of light through the preselected location of the target. The head carries means for generating a beam of light and means for receiving and transmitting the reflected light. Accordingly, with the head in the operative position, a density reading is immediately obtainable. The advantage of head in the retracted position, it does not interfere with location of the test region.

The housing, which is opaque, has an aperture through which the beam of light can project. It is important that the aperture be in registry with the preselected location of the target when the head is in the operative position. To accomplish this, the target includes means defining a recess with the preselected location being in the recess. The head includes a projection receivable in the recess in the operative position. In construction, the head is mounted on the supporting structure. With the head in the operative position the target is moved until the projection nests in the recess and then the target is affixed to the supporting structure. This properly orients the target.

The head can advantageously be moved between the operative and retracted positions if the head is pivotally mounted on a supporting structure. A linkage including a member pivotally mounted on the supporting struc: ture can be used to advantage to move the head from the retracted to the operative positions and biasing means can be used to automatically return the head to the retracted position.

The head preferably includes an opaque housing for excluding ambient light, a light source in the housing for providing a beam of light, light sensitive means responsive to the intensity of the reflected light for providing a signal, and the necessary filters. The present invention virtually eliminates the prior art optical systems in providing only a single lens. This lens is located in the path of the reflected light. The remaining portions of the reflection densitometer, including the necessary circuitry, are located outside of the head.

For color reflection densitometry this invention uses the characteristic that a first color may be defined as the absence of a second color. For example, yellow, magenta, and cyan are the absence of blue, green and red, respectively.

When light is directed against a test region of the first color, all of the visible spectrum is reflected except the second color. The second color is absorbed to the extent that the first color is present at the test region and is reflected to the extent that the first color is not present. A filter'of the second color is placed in the path of reflected light so that only the second color reaches the light sensitive means. Thus, a low intensity at the light sensitive means indicates that the first color at the test region is dense.

To enable the color reflection densitometer to measure the density of different colors, a filter carrieris mounted for movement on the housing of the head. Several different color filters are mounted on the filter carrier and are selectively movable by the filter carrier into the path of the reflected light.

If the reflection densitometer is to be used for test regions which reflect infrared, then an infrared rejection filter is employed in the beam of light to reject infrared. If infrared were not rejected, such test regions would reflect it. As the infrared would pass through any of the color filters, it would reach the light sensitive means and effect the signal provided thereby regardless of the density of the test region. A preferred infrared filter in-' cludes heat absorbing glass with a hot mirror dielectric coating.

The light sensitive means, which may be a photo transistor, provides a signal which is a function of the inten- 3 sity of the light striking it. This signal is processed electronically to provide a meter reading which may be representative of the density of the test region.

The invention can best be understood by reference to the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a reflection densitometer constructed in accordance with the teachings of this invention.

FIG. 2 is a side elevational view of the reflection densitometer with the head in the retracted position.

FIG. 2a is a side elevational view similar to FIG. 2 with the head in the operative position.

FIG. 3 is an enlarged sectional view taken generally along line 3-3 of FIG. 1 with the head in the retracted position.

FIG. 3a is an enlarged sectional view similar to FIG. 3 with the head in the operative position.

FIG. 4 is a longitudinal sectional view along the central axis of the head. i

FIG. 5 is a bottom plan view of the filter carrier taken generally along line 55 of FIG. 4.

FIG. 6 is a schematic view of a preferred form of electronic circuit for providing a density reading.

FIG. 7 is a schematic view for defining the reading angle and the lamp angle and is a front elevational view of the planes shown in FIG. 7a.

FIG. 7a is a perspective, schematic view illustrating the plane defined by the axes of the two light transmission conduits and its relationship to a surface, the density of which is to be determined.

FIG. 7b is an end elevational view of the two planes shown in FIG. 7a.

FIG. 8 is a graph of lamp angle versus differential density in percent of full scale deflection for several different reading angles.

FIG. 9 is a graph of reading angle versus differential density in percent of full scale deflection for several different lamp angles.

FIG. 10 is a graph of plane angle versus differential density.

FIG. 11 is a graph of plane angle versus sensitivity.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. l-3a show a densitometer ll constructed in accordance with the teachings of this invention. Generally, the reflection densitometer 11 includes a supporting structure 13, a target 15 carried by the supporting structure. and a head 17 pivotally mounted on the supporting structure for movement between a retracted position (FIGS. 2 and 3) and an extended position (FIGS. 2a and 3a).

In the embodiment illustrated, the supporting structure I3 is constructed of a molded plastic material. The supporting structure 13 may be of any suitable configuration for providing a frame or mounting structure for the various portions of the reflection densitometer.

The supporting structure 13 includes a target carrying section 19 to which the target 15 is mounted in any suitable manner such as by an adhesive. The target carrying section 19 and the target 15 are located at the forward end of the reflection densitometer 11. The section 19 projects upwardly from the target 15 to protect the latter and is opaque so as to assist in excluding light from the head 17 when the latter is in the operative position.

The target 15 in the embodiment illustrated is transparent and has cross hairs or indicia 21 thereon to facilitate the use of the target as locator means. The target 15 is flush with the lower surface of the densitometer 11 as shown in FIGS. 3 and 3a. The target 15 has an annular rib 23 which defines a recess 25. The recess 25 has tapered walls 27, and an aperture 29 is located centrally in the recess 25.

The head 17 is mounted for pivotal movement by pins 31 (only one being shown in FIGS. 3 and 3a) on upstanding arms 33 (only one being shown in FIGS. 3 and 3a) of the supporting structure 13. The head 17, which is described in greater detail below, includes a head housing 35, and the pins 31 may be molded integrally with the housing 35. In the embodiment illustrated, the housing 35 is formed from substantially indentical, molded, plastic half sections 37 and 39 as shown in FIG. 1. The housing 35 has a projection 41 of a configuration to be at least partially received within the recess 25 and to mate therewith. An aperture 43 is formed in the housing 35 centrally of the projection 41. The housing 35 is opaque so as to substantially exclude ambient light. The head 17 is normally biased to the retracted position (FIGS. 2 and 3)'by a pair of torsion springs 45 (only one being shown in FIGS. 3 and 3a) which are mounted on the pins 31, respectively. The springs 45 bear at one end on the interior of the head 17 and at the other end on a wall 47 of the supporting structure 13.

To move the head 17 to the operative position (FIGS. 2a and 3a) the housing 35 is provided with a pair of pins 49 (only one being shown in FIGS. 3 and 3a) which project laterally outwardly of the housing 35 in opposite directions. One of the pins 49 is engaged by an arm or crank 51 which is pivotally mounted on a tab 53 of the supporting structure 13 by a pin 55. The crank 51 has a cam surface 57 for engaging the pin 49. A similar arm or crank (not shown) is mounted on the supporting structure in an identical manner for cooperation with the other of the pins 49.

A handle or cover member 59 is mounted by pins 61 to webs 63, respectively, of the supporting structure 13. The cover member 59 forms a partial enclosure and cooperates with the supporting structure 13 to form a housing for portions of the densitometer 11. The interior of the cover member 59 is engageable with the upper ends 65 of the arms 51.

The reflection densitometer ll alsoincludes a meter 67 for providing density or other suitable readings. A knob 69 (FIG. 1) is used to zero the meter 67.

In the retracted position of the head 17, the longitudinal axis of the head is generally perpendicular to the plane of the target 15, and the axis of the aperture 43 is oriented approximately from the axis of the aperture 29. To move the head 17 to the operative position,

the user depresses the cover member 59. This pivots the cranks 51 clockwise as viewedin FIGS. 3 and 3a. Cooperation between the cam surfaces 57 and the pins 49 pivots the head 17 about the pins 31 against the biasing action of the spring 45. The head 17 is'pivoted approximately 90 to the position shown in FIGS. 2a and 3a. In this position, about one half of the head is received in the target carrying section 19, and the section 19 is closely adjacent three sides of the head 17. In the operative position, the apertures 29 and 43 are axially aligned and the projection 41 is substantially received within the recess 25.

One important function of the target is in the location of a test region. The reflection densitometer can be manually moved over a surface 71 (FIGS. 3 and 3a) with the head-17 in the retracted position until the aperture 29 is aligned with, Le, immediately over, a selected test region. The surface 71 may be paper and the test region may be an ink deposit on the paper. The head 17 is then moved to the operative position, and a density reading is taken as more specifically described below. Because the head is substantially received within the targetcarrying section19 and because the projection 41 mates with and contacts the surface 27, ambient light is excluded from the interior of the head.

It is important to the proper orientation of the reflection densitometer 11 that the apertures 29 and 43 be precisely aligned in the operative position. Accordingly, during construction of the densitometer 11, the head 17 is first mounted on the supporting structure 13 and then moved to the operative position. The target 15 is then placed on the supporting structure 13 and moved until the recess 25 receives the projection 41 of the head. The target 15 is maintained in this position while it is being adhesively or otherwise permanently secured to the supporting structure 13.

The target 15 need not be entirely transparent. However, the aperture 29 must readily pass light. In the retracted position, the head 17 leaves the target 15 substantially completely uncovered so as to facilitate use of the target as a locating means.

In a preferred construction, the head 17 takes the form shown inFIGS. 1-5. FIG. 4 shows the half section 37 of the housing 35, it being understood that the half section 39 is substantially identical with and mates with the housing section 37. The housing section 37 includes a bottom wall 73. a top wall 75, an end wall 77, and a side wall 79. The pins 31 and 49 are molded integrally with the side wall 79. The lower wall 73 also defines the projection 41, the outer surfaces of which are tapered to mate with the surface 27 of the recess 25.

The housing section 37 has a partition 81 of rather intricate shape extending from the bottom wall 73 to the top wall 75. The end of the housing section 37 opposite the end wall 77 is open. A plurality of holes 83 are molded in the walls 73 and 75 and corresponding pins (not shown) are molded in the housing section 39 for receipt in, and cooperation with, the holes 83 to attach the two housing sections.

A bushing 85 extends between an upwardly opening socket 86 in the lower wall 73 and a downwardly opening socket 88 formed by the partition 81. A filter wheel 87 is mounted on the bushing 85 for rotation. A peripheral portion 89 of the filter wheel 87 projects outwardly through a slot 91 in the end wall 77 to facilitate manual rotation ofthe filter wheel. The slot 91 is ofa minimum size to accommodate the length and thickness of the peripheral portion 89. As shown in FIG. 4, the filter wheel 87 extends between the partition 81 and the bottom wall 73.

A light source in the form of an incandescent lamp 93 ismounte'd within the bushing and a pair of leads- 95 for supplying electrical power to the lamp 93 extend out of the bushing 85 through a small aperture 97 in the partition 81. An infrared rejection filter 99 is mounted in the bushing 85 and in the socket 86 immediately beneath the lamp 93. The filter 99 has high transmittance in the visible range and very. low transmittance in the infrared range. In a preferred form the filter 99 is con structed of heat absorbing glass such as the glass known as KG3 which is available from Rolyn Optical in Areadia, California, having a very thin film of hot mirror dielectric.

A beam of light is transmitted from the lamp 93 to the aperture 43 through a light transmission conduit 101 which is molded in the bottom wall. The conduit 101 is coaxial with the lamp 93 and with the rotational axis of the filter wheel 87. The inner surfaces of the light transmission conduit 101 have a black mat surface beam of light from the lamp 93. The conduit 101 termi-v nates in a downwardly opening well 103 the outer end of which contains the aperture 43. The well 103 and the conduit 101 are of circular cross section with the well having the larger diameter and being coaxial with the conduit 103. The well 103 is defined by an annular flange 104 which is sized to be received in the aperture 29. In the operative position, the flange 104 is received in the aperture 29. If the densitometer 11 is to be used only for black and white, the flange 104 can be eliminated, the aperture 29 can be of the same diameter as the conduit 101, and the surface of the aperture 29 opaqued.

A conical reflection light transmission conduit 105 is also formed in the bottom wall 73 and extends from the well 103 upwardly asviewed in FIG. 4. The axes of the conduits 101 and 105 intersect in the plane at the outer end of the well 103, i.e., at the surface 71. The inner surface of the conduit 105 has a black mat finish to eliminate reflection, and the conduit 105 diverges as itextends upwardly. A lens 107, which, in the embodiment illustrated, is a duoconvex lens, is mounted in a socket in the conduit 105.

The filter wheel 87 has an inclined passage 109 therethrough which is coaxial with the conduit 105 and which is in the path of the reflected light transmitted by the conduit 105. A filter 111 is mounted in the passage 109. The nature of the filter 111 will depend upon whether the densitometer 11 is used for black and white or color. If the densitometer 11 is used for black and white, the filter 111 may be a neutral density filter which attenuates all wave lengths equally. If the densitometer 11 is used for color, the filter 111 may include a color filter and a neutral density filter for all colors except a blue filter.

A photo transistor 113 is mounted in the partition 81 with its photo sensitive area confronting the filter 111 and coaxial withthe conduit 105. The transistor 113 provides asignal which increases with an increase in the intensity of the reflected light. The transistor 113 is sensitive to infrared.

Assuming that the aperture 43 is confronting an appropriate test region on the surface 71, light from the lamp 93 is directed through the conduit 101 and the well 43 onto the test region. Substantially all of the light, except the light rejected by the infrared rejection filter 99, is either absorbed or reflected by the test region on the surface 71. The portion of the light which is reflected passes through the conduit 105 andthe lens 107 which focuses the light on the sensitive area of the photo transistor 113. The lens 107 serves to amplify the reflected light. The photo transistor 113 provides a signal as more particularly described in connection with FIG. 6 which is a function of the intensity of the reflected light. The circuit of FIG. 6 processes this signal and provides a reading on the meter 67 (FIGS. 1 and 3).

The infrared rejection filter is needed for all test regions that reflect infrared. Accordingly, the infrared rejection filter 99 must be used for all colors and for some black and white test regions. Many black test regions will absorb infrared, and therefore, for these test regions, the filter 99 is not required. 1

The filter wheel 87'includes a filter carrier 115 containing a plurality of filters 111, 111a, 1111) and lllc and an open passage 111d. For example, for color densitometry, the filters 111, 111a, lllb and 1110 may be blue, green, red and neutral density, respectively. In this example, the filters 111, 111a, lllb and lllc would be used to measure the density of yellow, red, blue and black test regions, respectively. If the densitometer is to be used only for black and white, then the filter wheel 87 can be locked in the appropriate positron.

Assuming that the density ofa yellow test region is to be determined, the filter wheel 87 is rotated to bring the filter 111 into registry with the conduit 105 as shown in FIG. 4. A suitable detent (not shown) releasably retains the filter wheel 87 in the selected angular position. As the color yellow is the absence of the color blue, the yellow test region will reflect all of the visible spectrum except blue. The amount of blue reflected by the test region is a function of the density or yellowness" of the test region. The blue filter 111 will reject all of the visible spectrum except for blue and, accordingly', only blue light reaches the sensitive area of the photo transistor 113. The intensity of the blue light at the photo transistor 113 is representative of the density of the test region. Specifically, the greater the intensity of the blue light at the photo transistor 113, the less dense is the test region. The density of other colors which can be defined as the absence of a certain color can be similarly measured.

It would be appreciated from the foregoing that, for color densitometry, an ability to differentiate between different densities in a very narrow band of the visible spectrum is very important. The present invention has such an ability to differentiate.

FIG. 6 shows one form of circuit which may be used to process the signal from the photo transistor 113.

FIG. 6 includes a battery 117 which may be mounted on the supporting structure 13 with the lamp 93 and a recharging jack 119 connected across the battery. A variable resistor 122 in series with the lamp 93 can be used, if desired, to adjust the lamp intensity. When a manual switch 121 is closed, current is supplied from the battery 117 to the lamp 93 and to the collector of the photo transistor 113 which conducts and provides a signal to the base of a transistor amplifier 123 in accordance with the intensity of the light. The signal from the photo transistor 113 increases with an increase in light intensity and this signal is amplified by the transistor 123.

The emitter of the transistor amplifier 123 is coupled to ground through a transistor 125 and a variable resistor 127. The value of the resistance 127 can be manually adjusted by turning of the knob 69. The voltage at the emitter of the transistor 125 is a function of the voltage drop across the resistor 127, and accordingly a the voltage at the emitter of the transistor 125 can be adjusted by adjusting the variable resistor 127. The voltage at the base of the transistor and at the base of an identical transistor 129 equals the voltage at the emitter of the transistor 125 plus the constant baseemitter voltage drop of the transistor 125. The voltage at the emitter of the transistor 129 equals the voltage at its base less the base-emitter voltage drop, and accordingly the voltages at the emitters of the transistors 125 and 129 are identical. The transistors 125 and 129 form a current converter amplifier which converts current to voltage with adjustable gain.

The collector of the transistor 129 is connected in series to the meter 67 which may be a milliammeter, and the emitter of this transistor is coupled to series resistors 131 and 133 with the former being a variable resistor.

The reflection densitometer 11 is placed over a test region and the head 17 is placed in the operative position. The switch 121 is closed by the member 59 as shown in FIG. 3a when the member 59 is depressed to move the head to the operative position. This illuminates the the lamp 93 and light reflected from the test region strikes the photo transistor 113 as described hereinabove with reference to FIG. 4. This causes the photo transistor 113 to conduct and the resistor 123 to amplify the signal from the photo transistor. Accordingly, a current path is established from the battery 117 through the transistors 113, 123 and 125 and the resistor 127. This establishes the voltage at the emitter of the transistor 129 and determines the reading on the milliammeter 67. Assuming that this test region is a standard region, such as a press proof, of known density, the knob 69 can be turned to vary the resistance of the resistor 127 to zero the meter 67. Specifically, increasing the value of the resistance of the resistor 127 increases the reading on the milliammeter 67.

The reflection densitometer 11 is then moved to a test region of unknown density. The voltage at the emitter of the transistor 129 is then established at the new test region by the current flow through the transistor 125 and the resistor 127. Any difference in density whether positive or negative will be shown by the milliammeter 67. Specifically, if the voltage at the emitter 129 is raised, then the reading on the milliammeter 67 decreases.

The voltage at the emitter of the transistor 129 can be changed by varying the variable resistances 127 or 131 or by a different light intensity striking the photo transistor 113. The resistor 127 is set when the reflection densitometer 11 is being zeroed, and the variable resistor 131 is set as a part of factory calibration to compensate for any variations in beta for the transistors 125 and 129. Thus, once the densitometer 11 has been zeroed, only a change in reflected light intensity at the photo transistor 113 will cause the meter 67 to move from its zzero reading. The reflection densitometer 11 functions as a null comparator in that it nulls on a known standard or press proof.

The circuit of FIG. 6 is particularly adapted for detecting and providing readings representing very small increments of density changes such as occurs in color densitometry. Although the output is logarithmic, for these small increments, it can be assumed to be linear. Moreover, the scale may be relatively arbitrary in that a difference of, for example, 20 in the scale'between the press proof and the test region will be meaningful to the operator.

To adapt the circuit of FIG. 6 to allow the scale of the meter 67 to cover a broader range of densisites as may be desirable for black and white, a current dependent resistor such as a transistor is used to replace the variable resistor 127. In all other respects, this circuit would be identical to the circuit of FIG. 6. This transistor provides a high resistance at small current values with the result that the scale of the meter 67 is effectively broadened.

FIGS. 7l1 relate to certain geometrical parameters of the present invention. As shown in FIG. 4 and as represented diagrammatically in FIG. 7a, the axes of the conduits 101 and 105 intersect at the surface 71 to define a plane 139 which intersects the surface 71. The surface 71 is assumed to be planar. As shown in FIG. 7b, the plane 139 may be moved through a plane angle P relative to a reference line 141 drawn perpendicular to the surface 71.

The plane 139 contains a reference line 143 (FIG. 7)

which is perpendicular to the surface 71 when the plane 139 is perpendicular to the surface 71. Thus, when the plane 139 is perpendicular to the surface 71, the reference linel43 is coextensive with the reference line 141', however, if the plane 139 (FIG. 7a) is titled through a plane angle P (FIG. 7b) relative to the reference line 141, it is assumed that the reference line 143 moves with and remains in the plane 139. As shown in FIG. 7 the axes of the conduits 101 and 105 and the reference line 143 intersect the surface 71 at a common point 137.

The photo transistor 113 may be moved through a reading angle R relative to the reference line 143, and the lamp 93 may be moved through a lamp or light angle L which may be either positive or negative. As shown in FIG. 7, the lamp angle L is positive when it is on the same side of the reference line 143 as the photo transistor I13.

FIG. 8 is a plot of differential density in percent of full scale deflection versus lamp angle L for reading angles of 30, 35, 40, 45, 50 and 60 and FIG. 9 is a plot of differential density versus reading angle for lamp angles ofl, 0, 5, 10 and Differential density is a measure of the ability of a reflection densitometer to differentiate between different densities. Differential density can be determined by taking readings on the reflection densitometer from two test regions of known density, taking the difference between these rcadings, and then converting such difference to a percentage of full scale deflection. For example, if the scale is considered as reading from 0 to 100, readings can be taken from test regions at the upper and lower range of densities commonly used. For black and white, this may be, for example, densities of 0.05 and L55. With the meter calibrated from 0 to 100, the difference in the two density readings represents a percentage of full scale deflection. The plots shown in FIGS. 8 and 9 were obtained by adjusting lamp angles and reading angles as indicated byusirig test regions of known densities of 0.05 and 1.55. Virtually identical graphs could beobtained by the use of color test regions of known density in lieu of black and white.

It has been found that a range of reading angles from 4 to 70 and a range of lamp angles from 30 to +60 can be used. The above ranges are satisfactory for color densitometry but are a bit too large for desirable results lems, and the necessity for a high differential density are all considered, one preferred construction employs a lamp angle of approximately 0 and a reading angle of approximately 30, and such a construction is shown in FIGS. 1-4.

FIGS. 10 and 11 show the effect of varying the plane angle P, it being immaterial whether the plane angle P is positive or negative. From FIGS. 10 and 11, it can be seen that a plane angle P of 0 is optimum in that it provides maximum sensitivity and differential density. This is optimum also from the mechanical and packaging standpoint. As the plane angle P increases, both the differential density and the sensitivity obtained are reduced. A range of plane angle P of i 30 is acceptable for most applications and a range of i 15 is preferred.

More specifically, differential density on the graph of FIG. 10 has been derived in the same manner as described above with reference to FIGS. 8 and 9. With reference to FIG. 11, percent sensitivity is defined as the sensitivity with at 0 plane angle, a 0lamp angle, and a 30 reading angle.

Although many concepts and features of this invention have been described with reference to densitometers, these concepts and features are also applicable to many other instruments which are responsive to reflected light.

Although exemplary embodiments of the invention have been shown and described, many changes, modifications, and substitutions may be made by one having ordinary skill in the art without necessarily departing from the spirit and scope of this invention.

We claim:

1. An instrument for providing an indication of the intensity of the light reflected from a test region, comprising: I

a supporting structure;

a target carried by the supporting structure, said target having a preselected location which is adapted to be placed over said test region, said preselected location of said target being capable of passing light;

a head including means for providing a beam of light and means for transmitting reflected light;

means for mounting said head .on said'supporting structure for movement between a retracted position in which said head leaves said preselected location uncovered whereby' said preselected location of said target can be placed over said test region and an operative position in which said head overlies said preselected location so that it can direct said beam-of light therethrough and said light transmitting means communicates with said preselected location of said target to receive and trans mit light reflected from said test region; and

means responsive to a characteristic of the light reflected into said light transmitting means for providing an indication of the intensity of the light reflected from the test region.

2. An instrument as defined in claim 1 wherein said means for mounting said head pivotally mounts said head on said supporting structure, said instrument including linkage means for pivoting said head between said positions thereof.

3. An instrument as defined in claim 2 wherein said linkage includes an arm pivotally mounted on said supporting structure and drivingly interrelated with said head to pivot the latter and a member pivotally mounted on said supporting structure and drivingly interrelated with said arm to pivot the latter, said instrument including means for biasing said head toward at least one of said positions thereof.

4. An instrument as defined in claim 1 wherein said target includes means defining a recess with said preselected location being in said recess, said head including a projection receivable in said recess in said operative position.

5. An instrument as defined in claim 1 wherein said head includes a housing, a light in said housing for providing said beam of light, and light sensitive means in said housing responsive to the intensity of the reflected light for providing a signal, said means for providing an indication including means outside of said housing responsive to said signal for providing said indication.

6. An instrument as defined in claim 1 wherein said beam of light and the reflected light transmitted by said light transmitting means lie substantially in the same plane, the reflected light transmitted by said light transmitting means forming a reading angle with a reference line. said reading angle being in the range of from 28 to about 60, said reference line being is said plane and oriented so as to be generally perpendicular to said surface when said plane is perpendicular to said surface.

7. The instrument as defined in claim 1 wherein said target is rigidly affixed to said supporting structure and immovable relative thereto.

8. The instrument as defined in claim 6 wherein said head includes a housing and said means for providing a beam of light includes a lamp in said housing, a filter wheel circumscribing said lamp and mounted for rotation within said housing, a plurality of filters carried by said wheel and selectively interposable into the path of the reflected light.

9. An instrument as defined in claim 7 wherein said head is pivotable between said positions thereof, said head including a projection and said target having a recess, said projection being at least partially receivable in said recess in said operative position, said preselected location being within said recess.

10. A method for obtaining an indication of the intensity of light reflected from a surface, comprising:

directing a beam of light onto said surface with the surface reflecting at least some of the light;

providing a signal which is a function of a characteristic of the reflected light from said surface along a predetermined path;

said beam and said path lying substantially in the same plane, a location along said beam being closely adjacent a location along said path said plane forming a plane angle with a first reference line. said first reference line being substantially perpendicular to said surface, said plane angle being in the range of from about 0 to about 30 on either side of said first reference line; said path forming a reading angle with a second reference line substantially in said plane, said reading angle being in the range of from about 28 to about said beam forming a lamp angle with said second reference line, said lamp angle having a magnitude of from 0 to 60 on the same side of said second reference line as said path and from 0 to 30 on the side of said second referenceline opposite to said path; and said second reference line being oriented in said plane so as to be substantially perpendicular to said surface when said plane is perpendicular to said surface.

11. A method as defined in claim 10 wherein said reading angle is approximately 50.

12. A method as defined in claim 10 wherein said reading angle is approximately 30.

13. A method as defined in claim 10 wherein said lamp angle is approximately 10.

14. A method as defined in claim 13 wherein said reading angle is approximately 50.

15. A method as defined in claim 10 wherein said lamp angle is substantially 0, said reading angle is substantially 30, and said plane is substantially perpendicular to said surface.

16. An instrument for providing an indication of the intensity of the light reflected from a surface, comprismg:

a housing;

means for mounting a lamp in said housing;

said housing having an aperture therein;

means defining a first light transmission conduit in said housing for directing the light from said lamp to said aperture and onto said surface;

means defining a second light transmission conduit in said housing leading away from said aperture for transmitting the light which is reflected by said surface, said light transmission conduits extending at an angle relative to each other;

an infrared filter in said first light transmission conduit intermediate said light and said aperture, said infrared filter including heat absorbing glass with a thin hot mirror dielectric coating;

a rotatable member within said housing;

a lens mounted in said second light transmission conduit intermediate said aperture and said rotatable member;

a plurality of filters carried by said rotatable member, said filters being selectively interposable in the path of reflected light transmitted by said second light transmission conduit, at least one of said filters being a color filter;

means for providing an indication of a characteristic of the light reflected through said second light transmission conduit; and

the light reflected through said second light transmission conduit forming a reading angle with a reference line perpendicular to said surface of from 28 to about 60.

17. An instrument as defined in claim 16 wherein the rotatable member substantially circumscribes the lamp and the light from said lamp forms a lamp angle with said reference line of substantially zero degrees. 

1. An instrument for providing an indication of the intensity of the light reflected from a test region, comprising: a supporting structure; a target carried by the supporting structure, said target having a preselected location which is adapted to be placed over said test region, said preselected location of said target being capable of passing light; a head including means for providing a beam of light and means for transmitting reflected light; means for mounting said head on said supporting structure for movement between a retracted position in which said head leaves said preselected location uncovered whereby said preselected location of said target can be placed over said test region and an operative position in which said head overlies said preselected location so that it can direct said beam of light therethrough and said light transmitting means communicates with said preselected location of said target to receive and transmit light reflected from said test region; and means responsive to a characteristic of the light reflected into said light transmitting means for providing an indication of the intensity of the light reflected from the test region.
 2. An instrument as defined in claim 1 wherein said means for mounting said head pivotally mounts said head on said supporting structure, said instrument including linkage means for pivoting said head between said positions thereof.
 3. An instrument as defined in claim 2 wherein said linkage includes an arm pivotally mounted on said supporting structure and drivingly interrelated with said head to pivot the latter and a member pivotally mounted on said supporting structure and drivingly interrelated with said arm to pivot the latter, said instrument including means for biasing said head toward at least one of said positions thereof.
 4. An instrument as defined in claim 1 wherein said target includes means defining a recess with said preselected location being in said recess, said head including a projection receivable in said recess in said operative position.
 5. An instrument as defined in claim 1 wherein said head includes a housing, a light in said housing for providing said beam of light, and light sensitive means in said housing responsive to the intensity of the reflected light for providing a signal, said means for providing an indication including means outside of said housing responsive to said signal for providing said indication.
 6. An instrument as defined in claim 1 wherein said beam of light and the reflected light transmitted by said light transmitting means lie substantially in the same plane, the reflected light transmitted by said light transmitting means forming a reading angle with a reference line, said reading angle being in the range of from 28* to about 60*, said reference line being is said plane and oriented so as to be generally perpendicular to said surface when said plane is perpendicular to said surface.
 7. The instrument as defined in claim 1 wherein said target is rigidly affixed to said supporting structure and immovable relative thereto.
 8. The instrument as defined in claim 6 wherein said head includes a housing and said means for providing a beam of light includes a lamp in said housing, a filter wheel circumscribing said lamp and mounted for rotation within said housing, a plurality of filters carried by said wheel and selectively interposable into the path of the reflected light.
 9. An instrument as defined in claim 7 wherein said head is pivotable between said positions thereof, said head including a projection and said target having a recess, said projection being at least partially receivable in said recess in said operative position, said preselected location being within said recess.
 10. A method for obtaining an indication of the intensity of light reflected from a surface, comprising: directing a beam of light onto said surface with the surface reflecting at least some of the light; providing a signal which is a function of a characteristic of the reflected light from said surface along a predetermined path; said beam and said path lying substantially in the same plane, a location along said beam being closely adjacent a location along said path said plane forming a plane angle with a first reference line, said first reference line being substantially perpendicular to said surface, said plane angle being in the range of from about 0* to about 30* on either side of said first reference line; said path forming a reading angle with a second reference line substantially in said plane, said reading angle being in the range of from about 28* to about 70*; said beam forming a lamp angle with said second reference line, said lamp angle having a magnitude of from 0* to 60* on the same side of said second reference line as said path and from 0* to -30* on the side of said second referEnce line opposite to said path; and said second reference line being oriented in said plane so as to be substantially perpendicular to said surface when said plane is perpendicular to said surface.
 11. A method as defined in claim 10 wherein said reading angle is approximately 50*.
 12. A method as defined in claim 10 wherein said reading angle is approximately 30*.
 13. A method as defined in claim 10 wherein said lamp angle is approximately + 10*.
 14. A method as defined in claim 13 wherein said reading angle is approximately 50*.
 15. A method as defined in claim 10 wherein said lamp angle is substantially 0*, said reading angle is substantially 30*, and said plane is substantially perpendicular to said surface.
 16. An instrument for providing an indication of the intensity of the light reflected from a surface, comprising: a housing; means for mounting a lamp in said housing; said housing having an aperture therein; means defining a first light transmission conduit in said housing for directing the light from said lamp to said aperture and onto said surface; means defining a second light transmission conduit in said housing leading away from said aperture for transmitting the light which is reflected by said surface, said light transmission conduits extending at an angle relative to each other; an infrared filter in said first light transmission conduit intermediate said light and said aperture, said infrared filter including heat absorbing glass with a thin hot mirror dielectric coating; a rotatable member within said housing; a lens mounted in said second light transmission conduit intermediate said aperture and said rotatable member; a plurality of filters carried by said rotatable member, said filters being selectively interposable in the path of reflected light transmitted by said second light transmission conduit, at least one of said filters being a color filter; means for providing an indication of a characteristic of the light reflected through said second light transmission conduit; and the light reflected through said second light transmission conduit forming a reading angle with a reference line perpendicular to said surface of from 28* to about 60*.
 17. An instrument as defined in claim 16 wherein the rotatable member substantially circumscribes the lamp and the light from said lamp forms a lamp angle with said reference line of substantially zero degrees. 